In the bonding of silicon wafers or semiconductor substrates (in the following: substrates), two substrates conventionally constructed as flat, circular discs—depending on the bonding process—are bonded to one another with a bonding layer, such as, for example, with adhesive (BCB, metal, solder) or without a bonding layer (SOI bonding or SDB bonding or anodic bonding) before further working steps.
Because of the pressure required and the heating up and cooling down operations required, hard holding or pressure plates made of metal or alloyed with metal have hitherto been used to bond the substrates to one another.
In this context, the substrates are loaded on to a lower holding plate in a process chamber. After the process chamber has been closed —depending on the bonding process—the process chamber is evacuated, if appropriate, and refilled with nitrogen, and then runs through a given combined pressure and temperature programme over a given dwell time of the wafers.
The chamber is then cooled to the unloading temperature (typically between 20 and 300° C.) and the upper pressure plate is raised, so that the bonded substrates can be removed.
In this process, the upper pressure plate, via which heating is also conventionally carried out, and the lower holding plate, which can likewise contain heating elements, are kept in contact with the substrates. Application of a contact force in combination with changes in temperature (heating up and cooling down) causes impairments and contamination of the substrates. Thus, for example, scratches are caused on the substrate reverse side (side facing away from the other particular substrate) due to different thermal expansion of the substrates compared with the contact surfaces of the pressure plate or holding plate.
Furthermore, particles can be transferred from the pressure plate or holding plate to the substrates.
The substrates are moreover contaminated by metal ions contained in the holding plate and/or pressure plate, since the intensive contact can effect atomic transfer of metal ions to the substrate.
In particular during curing of the optional bonding layer between the wafers under an applied pressure, stresses may be caused in the bonding layer, which can lead to distortions of the structures on the substrate and/or substrates in the μm range.
Because of the scratches and particles, an expensive cleaning (over-etching) following the bonding operation is often necessary, in order to be able to use the substrate further in the process line (in a clean room).
Metal ions can diffuse into the semiconductor crystal during subsequent oven processes and adversely influence properties thereof as a semiconductor.